October 18, 2012

Neandertal-modern hybrid babies and their heads

A discussion in the comments of Gene Expression got me thinking about a potential scenario for modern-Neandertal interbreeding dynamics. That discussion dealt with difficulties in childbirth arising from population differences in birth canal/head size.

The main idea is simple, and I will rephrase it as follows: offspring of a big man and small woman will tend to have bigger heads relative to the size of the woman's birth canal. On the other hand, offspring of a small man and big woman will not have that problem.

We have some information about differences between Neandertals and modern humans. The former were shorter and more "lateral" skeletally, while UP modern humans appear to have been more linear and taller. Headwise, modern humans had more globular head shapes, while Neandertals more linear ones, with no major differences in brain size between the two species.

If the above are correct, then male Neandertal-female modern human pairings would have a potential problem. Birth is a complex process, but at the end of the day, the most important factor is probably whether the diameter of the head can "fit" in the birth canal: the more it does not fit, the more likely it would seem that a mishap for both mother and offspring would occur.

Combine elongated Neandertal heads with narrow modern human pelves, and you have a potential problem. I am not 100% sure that modern humans and Neandertals differed in pelvis shape, although it would be a reasonable consequence of their overall build, but the same pattern would occur if they did not, simply on account of their different head shapes.

An additional factor involves sexual dimorphism, since male babies tend to be larger than female ones, and so any problems associated with "parental mismatch" might be particularly troublesome for male births.

So, all in all, we have 4 different cases:

Male H. n + Female H. s. => Male hybrid

Male H. n + Female H. s. => Female hybrid

Male H. s + Female H. n. => Male hybrid

Male H. s + Female H. n. => Female hybrid

It would appear, on the basis of the preceding discussion, that 1-2 would be more troublesome than 3-4, and 1 most troublesome of all. On the other hand, 4 seems to be the most advantageous case.

The most interesting thing about modern-Neandertal admixture is that it seems to have left no traces in uniparental markers, and, indeed, the lack of mtDNA lineages of Neandertal origin has been used to argue against the plausibility of estimated Neandertal admixture percentages.

If my reasoning is correct, then case #4 is particularly worrying, since female hybrids with Neandertal mtDNA ought to be the most easy to bear, and would also be the ones who would contribute Neandertal mtDNA in a mixed population.

On the other hand, case #1 would explain the lack of Neandertal Y-chromosomes, since crossings between male Neandertals and female modern humans that produce male offspring might be particular troublesome, and they would also be the ones to introduce Neandertal Y-chromosomes in the population.

Of course, we don't know enough about the dynamics of the admixture process; it might be possible that other factors influence the abundance of the four cases, both biological and cultural. For example, if modern humans had a behavioral advantage, then modern males may contribute most admixture, and this would make the worrying case #4 even more difficult to explain. On the other hand, how did the admixture take place? bride-stealing vs. rape would result in potential offspring being raised in different groups (father's vs. mother's), and there may also have been unknown cultural taboos involving admixture and offpsring produced from it.

In any case, this brief excursus may be useful for anyone thinking of writing some palaeo-fiction set in the Upper Paleolithic, and I'd love to hear from people who have data at hand that might be pertinent to the above discussion.

Personally I dont think frontier folk would have had any social problem with such matings. So they were a bit hairier and wider. I think they would not have been considered all that different. A different tribe, yes.

Are you censoring my comments on Human Hybridization and the Neanderthal Theory, for the purpose of stealing my ideas and taking credit for them if they should be proven correct? I believe so, because there's a pattern there.

Are you censoring my comments on Human Hybridization and the Neanderthal Theory, for the purpose of stealing my ideas and taking credit for them if they should be proven correct? I believe so, because there's a pattern there.

I am deleting comments in violation of blog rules, since this is my blog, and I decide which comments are acceptable and which are not.

You are of course free to make your own blog and post whatever you want in it.

Dienekes, you might may be interested in http://www.sciencedirect.com/science/article/pii/S0960982210012820:

"Comparing endocranial development between chimpanzees and modern humans from birth to adulthood, we have recently found that from the eruption of the deciduous dentition to adulthood the patterns of shape changes are remarkably conserved [7]. However, the developmental patterns differ markedly in the period directly after birth: within the first year of life, only modern human endocasts change rapidly from an elongated to a more globular shape. Notably, the shape changes during this ‘globularization phase’ in Homo sapiens seem to mirror the adult shape differences between brain cases of modern humans and Neanderthals."

"Three lines of evidence suggest that Neanderthals did not have a globularization-phase after birth. First, both Neanderthal and modern human neonates have relatively elongated braincases at the time of birth (Figure 1A), but only modern human endocasts change to a more globular shape between dental age groups 1 and 2 (see Supplemental Information, published with this paper online). By contrast, the endocranial shapes of the two youngest Neanderthal specimens in our sample, the neonate Le Moustier 2 (dental age group 1) and Pech de l'Azé (dental age group 3), are so similar that their reconstruction distributions (which reflect the estimation uncertainty) overlap (Figure 1B). Second, if Neanderthals and modern humans had the same globularization-phase after birth, a Neanderthal neonate would need to have an extremely elongated neurocranium and a very poorly developed cerebellum (Supplemental Information). "

"This challenges the view that all morphological characteristics separating modern humans from Neanderthals are already established at the time of birth. However, our results are not incompatible with the findings reported by Ponce de León and colleagues [3], [4] and [5]: when measurements of the face and neurocranium are analyzed together, the human and Neanderthal trajectories appear to be roughly parallel [3] and [4] because at the time of birth the face of a Neanderthal is already larger than that of a modern human (Figure 1A)."

"Our estimates of the endocranial capacity of Le Moustier 2 (408-428 cc) are similar to those reported for the Neanderthalneonate from Mezmaiskaya [4] and corroborate the finding that brain volume around the time of birth was similar in Neanderthals and modern humans [4] and [5]. Our virtual reconstructions also confirm that many facial characteristics that separate Neanderthals from modern humans, in particular the size of the face relative to the braincase, are established prenatally [3], [4] and [5]. However, most endocranial shape differences develop postnatally. Around the time of birth, modern humans and Neanderthals have similar endocranial sizes and shapes, with Neanderthals only being slightly more elongated than modern humans (Figure 1). After the constraints on neonatal shape and size imposed by the shape of the birth canal of the female pelvis [4], [5] and [8] are relaxed, the two species develop along different pathways."

Still, although the birth brain size and shape differences are slight (compared to the adult shape differences), it might have been enough to make a difference?

As a general rule, neonate size is determined more by the mother's size than anything else. The child will quickly grow to the expected parental average, etc. After all, there are plenty of modern human couples with tiny women and huge men. They don't have trouble having babies. This holds true for animals too, where you can have an even bigger difference in size.

Head circumference is the important factor, not length. (Circumference when squished up.) And it's not all about the head, shoulders count too! Body length is irrelevant as well.

Concerning lack of Neanderthal Y or mtDNA: Archaic introgression is assumed to have been limited [to how many individual matings?]. Advancing bachelor groups of AMH [or almost AMH] may have bequeathed hybrids to Neanderthal groups. Later, AMH family groups migrating into new territories may have allowed entry into their clan of a hybrid Neanderthal-AMH male who had localized hunting skills, superior strength etc. Y-DNA would be AMH; mtDNA not passed on. Head shape may not have been an issue in this scenario.

I knew a Doctor who worked in Saudi Arabia in the 80s in a region were the people are generally small & gracile ,he told me that women in this region had big problems giving birth, due to much larger Babies as a result of better nutrition of the mothers. So if modern humans have problems giving birth due to new life circumstances I can imagine that giving birth to a Hybrid would also cause complications.

If "better nutrition" is the risk factor that means the pregnant women did not have good nutrition at some point previously. Nutritionally-caused pelvic abnormalities (generally, rickets during the formative years for bones) are a well known risk factor for giving birth.

This is not a case of "bigger babies." It's a myth that babies that weigh more are harder to birth (outside of extremes). It's also a myth that many OBs tell pregnant women to get them to induce or have surgery before the baby is fully developed.

Head shape and size *is* a factor but, with same-species mothers and babies, you almost never have a baby "too big" to fit through the birth canal unless there is a deformity of the pelvic bones or some other unusual situation.

The easiest way to get no Neanderthal mtDNA in modern humans is to assume that AMH-Neanderthal couplings in both directions are predominantly short term rather than marriage-like (consistent with the archaeological absence of evidence of mixed tribes). If AMH mother hybrids end up in AMH tribes that don't go extinct, while Neanderthal mother hybrids end up in Neanderthal tribes that do go extinct, the mtDNA problem is resolved. Genetic drift early on could purge rare low frequency exceptions (a tiny share of even hybrid children).

The Y-DNA story is much harder to explain without something like the gender specific viability difference issues suggested in the OP or Haldane's Law (whose applicability has been questioned due to the relatively short period of differentiation between the two species/subspecies).

"A possible explanation is that very few Neanderthal males were accepted into the AMH tribes. Or that they were very much a minority and were drifted out."

It is tough to fit this explanation to the extreme divergence between a total lack of Neanderthal mtDNA and Y-DNA (out of a global sample size of individuals ever tested for mtDNA or Y-DNA haplogroup that is in the hundreds of thousand if not millions deliberately chosen to overrepresent rare and outlier populations and also hundreds of ancient DNA sample tested for mtDNA hg and dozens of ancient DNA samples tested for Y-DNA hg) and the observed levels of autosomal genetic admixture of Neanderthals with modern humans.

If one buy's my hypothesis about the absence of Neanderthal mtDNA being due to hybrid births being overwhelming in the tribe of the mother rather than being patrilocal, then no Neanderthal males were accepted into the AMH tribes at all, and all of the hybrids in the AMH tribes (and hence left descendants) were children of a Neanderthal male and an AMH female. Absent a mechanism to purge the Neanderthal Y-DNA one would naively expect a 1.25%+ Neanderthal Y-DNA haplogroup frequency.

With reasonable demographic assumptions is it exceedingly hard to get to 2.5%+ Neanderthal admixture by the point that introgressed Neanderthal DNA reaches fixation (particularly in light of the evidence that only a tiny minority of SNPs, mostly related to immune system function, conferred selective advantage), without any Neanderthal Y-DNA at all in this scenario by mere random chance. Only a few mechanisms can do this:

(1) Neanderthal Y-DNA purges within one to three generations (for reasons like reduced likelihood of a viable birth due to Haldane's Law or due to fetal head size making viable delivery unlikely), or

(2) there is some trait conferred by Neanderthal Y-DNA when combined with AMH DNA to reduce selective fitness in a hybrid individual raised in an AMH tribe (even a slight negative selective fitness effect will purge it over more than a thousand generations), or

(3) a lower than usually inferred effective population size (e.g. 400 or less) and a very low number of admixture events (e.g. 10 or less). The likelihood of Neanderthal Y-DNA drifting out while other autosomal Neanderthal admixture does not due to genetic drift (with reasonable demographic assumptions) is largely a function of the absolute number of admixture events and NOT the relative frequency of admixture events. The larger the absolute number of admixture events, the more than law of averages catches up with you and you get a low percentage of individuals with Neanderthal Y-DNA in the population that reaches fixation. Also, almost all off the drift of the Neanderthal Y-DNA out needs to take place in the first dozen or so generations, and disproportionately the earlier ones.

"then no Neanderthal males were accepted into the AMH tribes at all, and all of the hybrids in the AMH tribes (and hence left descendants) were children of a Neanderthal male and an AMH female. Absent a mechanism to purge the Neanderthal Y-DNA one would naively expect a 1.25%+ Neanderthal Y-DNA haplogroup frequency".

I agree with the possibility that 'no Neanderthal males were accepted into the AMH tribes', but it is quite possible that Y-DNA could be drifted out. It is uaually accepted they Y-DNA lines tend to be younger than mt-DNA lines in any particular region. Progressive replacement of Y-DNA could explain the lack of Neanderthal male lines' survival.

"the evidence that only a tiny minority of SNPs, mostly related to immune system function, conferred selective advantage"

Which very much supports the idea that survival of haplogroups is not a product of genetic, but something else. Presumably that 'something else' is associated with the surviving haplogroups. My suggestion is Y-DNA with technology and mt-DNA with culture.

"(1) Neanderthal Y-DNA purges within one to three generations (for reasons like reduced likelihood of a viable birth due to Haldane's Law or due to fetal head size making viable delivery unlikely)"

Even if fully inter-fertile the Neanderthal Y-DNA could be lost through drift in presumably small populations.

"(2) there is some trait conferred by Neanderthal Y-DNA when combined with AMH DNA to reduce selective fitness in a hybrid individual raised in an AMH tribe (even a slight negative selective fitness effect will purge it over more than a thousand generations)"

As I mentioned above, I don't think 'genetic fitness' has anything to do with it.

"(3) a lower than usually inferred effective population size (e.g. 400 or less) and a very low number of admixture events (e.g. 10 or less). The likelihood of Neanderthal Y-DNA drifting out while other autosomal Neanderthal admixture does not due to genetic drift (with reasonable demographic assumptions) is largely a function of the absolute number of admixture events and NOT the relative frequency of admixture events. The larger the absolute number of admixture events, the more than law of averages catches up with you and you get a low percentage of individuals with Neanderthal Y-DNA in the population that reaches fixation".

Although I see no reason for genetic selection in the case of haplogroups I believe it is quite possible that some Neanderthal a-DNA provided a genetic advantage in those populations with it. After all Neanderthals had been adapting to the local conditions for thousands of years, through a whole series of climate changes.

Old Blog Archive

Dienekes' Anthropology blog is dedicated to human population genetics, physical anthropology, archaeology, and history.

You are free to reuse any of the materials of this blog for non-commercial purposes, as long as you attribute them to Dienekes Pontikos and provide a link to either the individual blog entry or to Dienekes Anthropology Blog.

Feel free to send e-mail to Dienekes Pontikos, or follow @dienekesp on Twitter.